XPS and Raman spectroscopy were per

XPS and Raman spectroscopy were performed to evaluate the quality of the graphene powders, as shown in Fig. 2. The C1s/O1s values of G-A and G-B are 27.6 and 34.1, respectively, much higher than that of GO [46], indicating a relatively low degree of oxidation. As shown in the Fig. 2a and b, the XPS spectra of G-A and G-B, along with the peak of sp2 carbon that can be fitted with reference spectrum, the peaks attributed to the Csingle bondOH bond and Csingle bondOsingle bondC bond are also observed [47]. Similar conclusions can be obtained by Raman spectroscopy (Fig. 2c), where the raw graphite powders possess a typical spectrum reported by others [30], [48], [49] with the ID/IG ratio of ~0.06. The shoulder peak in 2D-band of graphite is not found at the G-A and G-B [50], [51], and the shape of 2D-band of G-A and G-B is quite similar to that of graphene ink reported by references [22], [26], indicating that the exfoliation degree of two kinds of graphene powders is high. The exfoliation also causes a little increase of ID/IG to 0.17 and 0.09 for G-A and G-B, respectively, but broadened G and D′ peak merging into one wide bond due to a large amount of defects presenting in the basal plane of graphene [52], [53] is not found. So the increasing ID/IG can be attributed to the graphene flake edges [22], to which the oxygen-containing functional groups are mainly attached [54]. From the results of XPS and Roman, it can be conclude that G-A and G-B possess few defects that will degrade the electrical conductivity of graphene flakes, and the higher oxygen content and ID/IG could be attribute to its smaller mean area and accompanying more flake edges which has already been certified by AFM results.Download : Download high-res image (299KB)Download : Download full-size imageFig. 2. (a), (b) C1s X-ray photoelectron spectra of G-A and G-B. (c) Raman spectra (514 nm) of graphite, G-A and G-B.3.2 The rheological properties of graphene/carbon black inksScreen printing conductive inks with suitable rheological properties are able to possess excellent printable properties, which can be transferred into thinner lines with smoother edges and be beneficial to eliminate defects and bubbles in the printed patterns making them possess higher electrical conductivity. Therefore, how the carbon black content and the properties of graphene powders influenced the rheological behavior of graphene/carbon black inks were investigated using a plate-plate rheometer. The viscosity at different shear rates from the steady-state flow step test was measured, as shown in Fig. 3a. It can be seen that the viscosity of inks declines as the shear rate increases, which is a typical performance of pseudoplastic fluid. Also, conductive carbon blacks with high electrical conductivity usually possess higher surface area and structures, and contain a significant volume of micropores [55], all of these will raise the difficulty of conductive fillers dispersion. Therefore, the conductive inks with higher carbon black content exhibit higher viscosity at the same shear rate before obvious sample spill which occurs about 1 s−1 for G-A-ink-15. The higher the shear rate the faster the paste is ejected, so the ink viscosity with carbon black is lower than that of G-A-ink-0 when shear rate exceeded a certain value, e.g. 26.2 s−1 for G-A-ink-15. Despite the same content of carbon black contained in conductive fillers, the ink viscosity of G-B-ink-15 is higher than that of G-A-ink-15. Since G-B involves higher percentage of thinner and larger-area graphene flakes, the degree of agglomeration against the dispersion of graphene powders will increase, thereby causing the increase of viscosity [56].